Nonionic block copolymers comprising blocks of polyoxypropylene and polyoxyethylene have been synthesized and shown to have variable uses depending the molecular size of the hydrophobic and hydrophilic regions. The commercially available nonionic copolymers are molecules that have low molecular weight hydrophobic polyoxypropylene regions with varying percentages of total molecular weight hydrophilic regions attached. These nonionic copolymers are prepared by the sequential addition of two or more alkylene oxides to a low molecular weight water-soluble organic compound containing one or more active hydrogen atoms.
The prior art methods of synthesizing nonionic block copolymers includes the sequential addition first of propylene oxide units and then ethylene oxide units to a low molecular weight, water-soluble organic initiator compound, such as propylene glycol. The oxyalkylation steps are carried out in the presence of an alkaline catalyst such as sodium or potassium hydroxide. The alkaline catalyst is then neutralized and removed from the final product. The size of copolymers made using this technique are limited to molecules with hydrophobic molecular weights of approximately 4000, 10 to 80% of the total molecule consisting of ethylene oxide.
Other nonionic copolymers have been synthesized using nitrogen containing molecules as the base molecule. The condensation of propylene oxide with a nitrogen-containing reactive hydrogen compound and the subsequent condensation of ethylene oxide therewith were carried out in the known manner for condensing alkylene oxides with reactive hydrogen compounds. The process is normally carried out at elevated temperatures and pressures in the presence sodium hydroxide, potassium hydroxide, sodium alkoxide, quartenary ammonium bases and the like. The condensation reactions can also be carried out in the presence of an acid catalyst. The manipulative steps will vary to some extent depending upon the normal physical state of the reactive hydrogen compound.
Although nonionic block copolymers can be synthesized with low molecular weight hydrophobic regions, using conventional alkali catalyzed polymerization methods, no one has been able to synthesize nonionic copolymers with high molecular weight hydrophobic regions. Problems with the synthesis of high molecular weight nonionic polyoxyethylene/polyoxypropylene copolymers, especially with high molecular weight hydrophobe regions, include a high degree of unsaturation and a high degree of premature chain termination resulting in a distribution of components with low molecular weight chains along with the distribution of components with desirable high molecular weight chains. Using prior art methods of producing polyoxyethylene/polyoxypropylene block copolymers results in an unacceptable variety of polymer sizes and an unacceptably high degree of unsaturation in the polymer. This is especially undesirable when the copolymers are to be used in biological application.
One of the needs of the medical industry is for compounds that modulate the immune response in various ways. In addition, compounds are needed to facilitate gene transfer in cells. For example, over the past decade, the emergence of methods of gene transfer to mammalian cells has prompted enormous interest in the development of gene-based technologies for the treatment of human disease. Current gene therapy technology has focused primarily on the use of viral and retroviral vectors which provide highly efficient transduction and high levels of gene expression in vivo. The most studied are retroviral vectors, replication-defective murine retroviruses, which require specialized "packaging" cell lines for their replication. Retroviral vectors integrate into chromosomes of dividing cells leading to stable expression of the integrated gene. Also, replication-defective adenoviral and adeno-associated viral vectors have been extensively utilized. These vectors have the advantage of efficiently transducing non-dividing cells, generally do not integrate into the host cell genome, and result in high levels of transient gene expression. However, the use of viral methods of gene transfer for human therapy has raised safety concerns mainly due to the potential of replication-defective viral vectors to become replication-competent and therefore infectious (reviewed by Mulligan, 1993).
An alternative to viral gene transfer has been the use of non-viral methods such as: cationic liposomes, delivery of ligand-DNA complexes by receptor-mediated endocytosis, DNA coated microprojectiles and naked DNA. Liposome-mediated gene transfer has been utilized extensively in in vitro transfection studies but its application for in vivo gene delivery has been limited. The main disadvantage of these methods is that only transient gene expression is achieved and thus repeated administrations would be necessary if continued gene expression were needed.
Recent studies have focused in the use of naked DNA for genetic immunization. It has been shown that intramuscular inoculation of BALB/c mice with a high concentration of plasmid DNA encoding influenza A nucleoprotein results in the generation of specific CTL responses and protection from a challenge infection of influenza A virus (Ulmer, J. B., et al. (1993) Heterologous protection against influenza by injection of DNA encoding a viral protein. Science 259, 1745-1749). Successful genetic vaccination against influenza virus has also being obtained by intradermal immunization with naked DNA (Raz, E. et al., (1994) Intradermal gene immunization: The possible role of DNA uptake in the induction of cellular immunity to viruses. PNAS 91, 9519-9523). Although successful immunization has been achieved using DNA alone, other more efficient methods of DNA delivery such as the use of DNA-coated microprojectiles are being explored (Vahlsing, H. L., et al. (1994) Immunization with plasmid DNA using a pneumatic gun. J. Immunol. Meth. 175, 11-22).
What is needed is a composition of polyoxyethylene/polyoxypropylene block copolymers with narrow molecular weight distribution and polyoxypropylene hydrophobic block molecular weight higher than approximately 7000. Further, what is needed is a method for synthesizing nonionic polyoxyethylene/polyoxypropylene copolymers with a narrow molecular weight distribution and high molecular weight polyoxypropylene hydrophobe. These copolymers should also have enhanced activity as adjuvants, permitting vaccination with lower amounts of antigens such as viral proteins, and display lower toxicity than conventional adjuvants. Also needed are compounds that can facilitate the transfer of genes to cells.